The invention relates to an improved process for the preparation of polyethers by the reaction of certain polyols with alkylene oxides. The resulting polyethers are useful as starting materials for the production of polyurethanes, in particular, rigid polyurethane foams.
The invention relates, more specifically, to the reaction of an alkylene oxide with a polyol reactant characterized by a high melting point temperature. Because of the relatively high melting points of certain such polyols, it is often desired to accomplish their reaction with alkylene oxide at a temperature near to or below the melting point of the polyols. The reaction then proceeds rapidly upon contact of the fluid alkylene oxide with the polyol reactant, for instance, a solid in particulate form. However, because of the highly exothermic nature of the reaction, the most straightforward approach to providing this contact, i.e., simply adding the alkylene oxide to a reaction vessel containing the polyol reactant, is complicated by rapid reaction at the particulate surface leading to scorching (charring) and caking of the polyol and excessive formation of reaction by-products. While vigorous agitation of the reaction mixture may to some extent alleviate these problems, as a practical matter effective agitation is difficult in a reaction vessel containing a high loading of the polyol.
It has been suggested that the desired reaction be carried out in an inert liquid reaction medium, for instance, in a xylene solvent. Performing the reaction in this manner is found, however, to considerably reduce the rate at which polyether is obtained. Reaction in a solvent medium also necessitates, of course, separation of the product from the solvent.
Attempts have further been made to perform the reaction in the presence of water or a low molecular weight polyhydroxy compound such as glycerol. Yet, the presence of water or glycerol reduces the functionality of the product (i.e., the average number of alkylene oxide molecules reacted with a molecule of the polyol). A relatively high functionality is known to be desirable for many applications of the product polyethers.